Diffusers of the type having closeable and openable vanes have been somewhat expensive to manufacture because of the number of parts required for their operation. Such diffusers have universally been made with the vanes having rotation in the same direction between their open and closed positions. In such structures, when the vanes are partly opened, the vanes have air directing dispositions which will cause air flow to be diverted in the direction of the vanes. In situations where it is desired to eliminate the air directing feature, alternate vanes must be rotated in opposite directions in order that air direction by the vanes will not occur.